The present invention relates to intake manifolds for internal combustion engines. More in particular, the present invention relates to a unique manifold that effects a wide band of high torque.
Intake manifolds of internal combustion engines transport combustion air to the cylinders of the engine for consumption. In a carbureted internal combustion engine, an intake manifold also transports fuel with the air. The carburetor typically mounts over a plenum of the manifold. The fuel and air mixture enters the plenum and from the plenum travels to the cylinders through ducts called runners. The runners exit at inlet ports of the engine. These ports lead to the cylinders through inlet valves.
A fuel and air mixture is drawn into each cylinder of an engine by a vacuum created there by downward piston movement during the inlet cycle of the cylinder, although this pressure differential that forces the fuel and air mixture into the cylinder may be augmented substantially by a turbo charger or a super charger. Inlet and exhaust valves into each cylinder provide for the admission of the fuel-air mixture into the cylinder and the exhaustion of products of combustion from the cylinder. These valves open and close every other revolution in a four-cycle engine, and do so gradually; that is, the valves do not open and close instantaneously but over several degrees of engine revolution.
The dynamics of induction of fuel and air into an engine are very complicated, making generalization difficult. The factors affecting induction include intake and exhaust valve timing, piston speed, inertia of gases undergoing induction, fluid friction, resonance, intercylinder interference, and induction geometry, to name a few.
The intake timing of today's internal combustion engines has an inlet valve opening while its companion exhaust valve is closing and before the piston reaches top dead center. An inlet valve closes several crank degrees after its piston reaches bottom dead center. This timing accommodates the several crank degrees of engine revolution required to effectively open and close an inlet valve. In other words, to have the inlet valve as open as possible during the descent of the piston, the inlet valve is given a head start and starts to open before the piston actually begins to descend. To have the inlet valve open and to take advantage of gas inertia, the inlet valve does not close until the piston has begun ascending in the cylinder again. Gas inertia is sufficient to overcome the adverse pressure caused by the ascending piston.
It is quite apparent that the more mixture that is inducted into a cylinder with each cycle, the more power an engine will have, and the more efficient it will be. The measure of engine efficiency reflecting the amount of cylinder charge is "volumetric efficiency" which is the volume of air a cylinder actually receives divided by the volume swept by the piston. If the air flowing into the runners of an inlet manifold is traveling fast, its inertia results in an additional amount of mixture charged into the cylinders and an increase in volumetric efficiency compared with the charge resulting from slower air. More specifically, the torque of an engine at relatively low engine speeds is enhanced by increasing the velocity of the mixture in the intake manifold, and this does not adversely affect the manifold's performance because the manifold would not be used at high engine speeds where induction loss could be significant.
Piston speed directly measures the pumping characteristics of an engine. The higher the piston speed the more mixture is inducted into the engine in a given time. Piston speed also generates pressure pulses that affect movement of the mixture in the intake manifold. As the piston descends, a negative pressure signal results and this signal travels upstream in the manifold. It is this negative pressure that produces induction. As the piston ascends, it produces a positive pressure signal that travels upstream in the manifold and opposes induction. The magnitude of the signals is a direct function of piston speed which varies even at constant engine speed. The pressure signals travel at the speed of sound; the mixture travels much slower. The pressure signals can be used to enhance volumetric efficiency. As a pressure pulse travels up a runner of a manifold and reaches atmosphere, which may be in the plenum of the manifold, the air there overcompensates for the disturbance caused by the pulse. Thus, when a pressure pulse traveling upstream is positive with respect to mean inlet manifold pressure, it initially compresses the air in the plenum and the air is pushed out of the way creating a locally rarified zone. The resulting negative pressure travels down the manifold and detrimentally affects the flow of gas in the manifold by reducing the pressure differential. Of more interest is a negative pressure pulse traveling upstream and produced by a descending piston. This negative pressure pulse will create a rarifaction in the plenum and air will rush in to fill the resultant depressed zone generating a positive pressure pulse that travels down the runner towards the cylinder. If the pulse arrives at the cylinder at the right time, say when the inlet valve is about to close, the pulse can add significant quantities of mixture to the cylinders to increase the power of the engine by increasing the volumetric efficiency of the engine. This is known as intake manifold tuning and obviously relies upon the resonance of the mixture which, practically speaking, means resonance of the air.
The speed of a pressure pulse is independent of manifold geometry. The velocity of the air in a manifold's runners, however, is not.
At open throttle, piston speed and runner cross-sectional area determine the mixture speed through a manifold. Mixture velocity generally is a direct, linear function of piston speed, at least at low engine speeds, with runner cross-sectional area held constant. Runner gas velocity is an inverse function of runner cross-sectional area; as the cross-sectional area of the manifold runner decreases, the air increases in velocity. As the length of the runner increases, the time required for a pulse to travel upstream and back downstream increases.
The pressure history of one cylinder in a multiple cylinder engine can affect the induction performance in other cylinders. Thus pressure pulses traveling up a runner from one cylinder can interfere with the pressure within other runners. Generally, it has been the practice to design inlet manifolds to eliminate intercylinder interference.
The isolation of cylinders in an internal combustion engine to avoid intercylinder affects on induction has taken different forms. One isolation technique in practice is the so-called two-plane, over and under, 180.degree. manifold. This manifold has been a standard for some time for most American production V-8 engines using a single four-barrel or two-barrel carburetor. It has two plenums. Each plenum does not directly communicate with manifold runners; instead, stubs between the runners and the plenum communicate the runners with the plenum. Runners sharing a stub are remote from each other in the sense of the engine's firing order. The plenums of the manifold are isolated from one another, and alternate runners in the sense of an engine's firing order go to alternate of the plenums. By isolating alternate cylinder combustion events from each other through the use of two plenums, intercylinder effects are attenuated.
An independent runner manifold, such as described in U.S. Pat. No. 3,744,463 to James D. McFarland, has a common plenum for all the cylinders of an engine directly communicating with the cylinders through an independent runner for each cylinder. The plenum employs no partitions to separate the plenum into two plenums. Independent runner manifolds in many applications have advantages over two-plane manifolds. The advantages inhere from simpler induction paths afforded by the manifold and include better cylinder-to-cylinder air-to-fuel ratio uniformity and lower pumping work.
Particularly with engines that have modest power outputs, it is important to have a broad band of relatively high torque so that engine performance over a range of engine speeds is good. With a narrow band of high torque engine performance at engine speeds out of the band may well be sluggish or mediocre, even though quite satisfactory within the band.